1. Field of Disclosure
The present disclosure relates generally to a phase locked loop (PLL) and specifically to calibration of a voltage controlled oscillator (VCO) for the cellular phone.
2. Related Art
Cellular phones have evolved from large devices that were only capable of analog voice communications to comparatively smaller devices that are capable of digital voice communications and digital data communications, such as Short Message Service (SMS) for text messaging, email, packet switching for access to the Internet, gaming, Bluetooth, and Multimedia Messaging Service (MMS) to provide some examples. In addition to these capabilities, the cellular phones of today have additional non-communication related capabilities, such as a camera with video recording, an MPEG-1 Audio Layer 3 (MP3) player, and software applications such as a calendar and a phone book, to provide some examples. Even in light of these capabilities, manufacturers of cellular phones are placing even more capabilities into cellular phones and making these more powerful cellular phones smaller.
At the heart of each cellular phone lies a phase locked-loop (PLL). The PLL is responsible for providing an appropriate transmit frequency for the cellular phone prior to commencement of a transmit mode of operation. The PLL is also responsible for providing an appropriate receive frequency for the cellular phone prior to commencement of a receive mode of operation. In order to properly provide the appropriate transmit and/or receive frequency for the cellular phone, a voltage controlled oscillator (VCO) located within the PLL is calibrated to the appropriate transmit and/or receive frequency of the cellular phone. Once the VCO is initially calibrated to be sufficiently proportional to a frequency and/or a phase of a reference frequency, the PLL locks the frequency of the VCO to be proportional to the frequency and/or the phase of the reference frequency to provide the appropriate transmit and/or receive frequency. Often, additional calibration of the VCO following the initial calibration is often required to ensure that the VCO is sufficiently proportional to the frequency and/or the phase of the reference frequency to the appropriate transmit and/or receive frequency. For example, an additional calibration may be required once the cellular phone transitions from the transmit mode of operation to the receive mode of operation and/or from the receive mode of operation to the transmit mode of operation.
Communications standards provide a certain time window for an initial calibration of the VCO. However, in certain communications standards often do not provide sufficient time for the additional calibration, which is typically necessary when the cellular phone transitions from the transmit mode of operation to the receive mode of operation and/or from the receive mode of operation to the transmit mode of operation. As a result, the cellular phone may not be aligned to the appropriate receive frequency when the cellular phone switches from the transmit mode of operation to the receive mode of operation, for example.
Thus, there is a need to calibrate the VCO following the initial calibration when transitioning from the transmit mode of operation to the receive mode of operation and/or from the receive mode of operation to the transmit mode of operation to the appropriate transmit and/or receive frequency of the cellular phone but to do so within the time allotted in the respective communications standard. Further aspects and advantages of the present disclosure will become apparent from the detailed description that follows.
The present disclosure will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.